The broadest objectives of our studies are to define the cellular and molecular basis of the immunological abnormalities with aging. It has already been found that studies in man revealed that T-lymphocytes from elderly persons were impaired in their response to PHA, PWM or allogenic lymphocytes. It is now our intention to identify the aspects in T cell differentiation which are comprised during aging. The first aspect will involve the estimation of the number of T cell precursors in old and young. The second phase will involve thymus-mediated differenthation and the assessment of the capacity of thymus glands from old and young mice to impose histocompatibility restriction on helpter T cells. The third phase will involve T-cell function with the effect of thymopoietin on the ease of T cell tolerance induction to be determined.